LED light sources are often used to replace low voltage (12V) halogen lamps in order to save energy, but problems may occur when LED light sources replace halogen lamps and are connected to existing electronic transformers.
Electronic transformers contain a high-frequency oscillator feeding an isolation transformer which powers a connected load, such as a halogen lamp or an LED light source. The load needs to draw sufficient power from the electronic transformer in order for the transformer to turn on and supply power, i.e. the transformer needs to be sufficiently loaded. Typically, electronic transformers for 12V halogen lamps are rated to operate with loads of 20-70 W and the halogen lamps are typically supplied with 20 W or 50 W power ratings. However, LED light sources that replace halogens typically draw far less power than 20 W, with the result that the transformers are under loaded.
Electronic transformers cease to supply power if the connected load is below a minimum level. When connected to an electronic transformer, LED light sources initially present the transformer with a sufficient load to turn the transformer on. However, following an initial inrush power, the LED light source draws less power than the minimum requirements of the electronic transformer (i.e. presents a low load) and hence the transformer turns off and ceases to supply power.
Depending on the particular electronic transformer, it may or may not turn on (re-ignite) again to supply power, with the result that the LED either receives no power (i.e. malfunctions completely) or the cycle described above is repeated and the LED flickers.
Most attempts to address the problem with under-loaded transformers are based on booster circuits followed by buck regulators. These booster circuits lower the impedance presented to the transformer. These circuits contain inductors, switches and some logic circuits like PWM generators, which cause the circuits to be costly and bulky. Further, the high switching causes EMI related problems. U.S. 20110115400 A1 discloses an example of such a booster circuit.
Other solutions attempt to ignite transformers by feeding pulsed currents back into the transformer in order to re-ignite the transformer. These solution also require logic circuitry and bulky capacitors in order to feed high enough current back to the transformer in order to re-ignite such transformer. EMI issues could occur as high current re-ignition pulses flow between LED light and transformer. U.S. Pat. No. 6,433,493 discloses an example of feed back to re-ignite a transformer.
The present invention seeks to keep electronic transformers functioning under low load conditions, to provide power to LED light sources, while avoiding shortfalls such as visible flickering, high costs, and bulk.